FAN7621B Fairchild Semiconductor, FAN7621B Datasheet - Page 13

• Manufacturer Part Number: FAN7621B
• Description: The FAN7621B is a pulse frequency modulation controller for high-efficiency half-bridge resonant converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN7621B • Rev. 1.0.1
6. PCB Layout Guideline: Duty imbalance problems
may occur due to the radiated noise from main
transformer, the inequality of the secondary-side leakage
inductances of main transformer, and so on. Among
them, it is one of the dominant reasons that the control
components in the vicinity of R
primary current flow pattern on PCB layout. The direction
of the magnetic field on the components caused by the
primary current flow is changed when the high-and-low
side MOSFET turns on by turns. The magnetic fields with
opposite direction from each other induce a current
through, into, or out of the R
on duration of each MOSFET different. It is strongly
recommended to separate the control components in the
vicinity of R
PCB layout. Figure 28 shows an example for the duty-
balanced case. The yellow and blue lines show the
primary current flows when the lower-side and higher-
side MOSFETs turns on, respectively. The primary
current does not enclose any component of controller.
In addition, it is helpful to reduce the duty imbalance to
make the loop configured between CON pin and opto-
coupler as small as possible, as shown in the red line in
Figure 28.
T
pin from the primary current flow pattern on
T
pin, which makes the turn-
T
pin are enclosed by the
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Figure 28. Example for Duty Balancing
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